Modal analysis of the full poloidal structure of the plasma response to n = 2 magnetic perturbations.

Previous DIII-D research has identified the presence of a multimodal plasma response to perturbations with toroidal periodicity equal to 2, with kink-like phenomenology of the plasma response in the low field side mid-plane and a resonant-like behavior in the high field side midplane. In this paper, we extend that initial midplane observation with the first detailed analysis of the poloidal structure of the plasma response as a function of the poloidal spectrum of the applied n = 2 perturbation and compare it to predictions made with the linear magnetohydrodynamics code MARS-F, confirming an overall good agreement. Singular value decomposition analysis of both the experimental data and the modeling identifies two distinct poloidal mode structures, one peaking at the low field side midplane and one peaking at θ ± 50°, each with a well-defined dependence on the poloidal spectrum of the applied perturbation. A study of the effect that βN has on the two modes confirms the kink-like phenomenology of the plasma response that dominates the low field side mid-plane, shows that the behavior previously associated with the high field side is observable at all poloidal angles, and highlights that the mode dominant at low βN becomes the secondary one at high βN. Detailed analysis of the high field side response is presented as well, suggesting the presence of a third mode linked to the presence of non-linear effects corresponding to particular poloidal spectra of the applied perturbation. [ABSTRACT FROM AUTHOR]